During plastics moulding operations, for example, blow-moulding, blanks or parisons are pre-heated using infra-red radiators which allow very short heating periods of only a few seconds. Infra-red heat treatment offers the advantage of heating not only the outer surface of the plastics material but also of directly reaching the material below the outer surface.
Nevertheless the thermal energy is absorbed to a large extent by the material closest to the heat source. This causes a high temperature peak on the surface of an article during intensive radiation. When heating blanks made of synthetic plastics material, the blanks have been treated, for example, in such a way that the material to be heated moves past the infra-red radiator approximately equidistantly from the infra-red radiator and a flat reflector.
Heating of the blanks is effected by direct radiation, indirect radiation and by convection. Convection is very high in this system, since a large proportion of the radiated energy is returned to the radiator by the flat, parallel reflector; therefore, the system retains a high temperature. Moreover, convection can lead to undesirable over-heating of some parts of the outer surface of a blank.
Because plastics materials are poor conductors of heat, the temperature balance within the walls of a hollow or solid blank takes place only relatively slowly during the short heating period for which infra-red radiation treatment is effected. When components with a large wall thickness have to be heated a little longer than usual so as to obtain the required amount of thermal energy, this often leads to undesirable changes in the surface layer, for example a change in the structure or configuration of the material due to overheating or the appearance of crystallisation phenomena in, for example, polyethyleneterephthalate, (PET).
An object of the invention is to avoid the disadvantages of known heating apparatus, and in particular to ensure that the required heating time for heating a component is shortened, to prevent overheating of the radiator by reflected radiation and thereby relieve the infra-red radiator to give it a longer life.